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<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
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  <title>Document</title>
</head>

<body>
  <script>
    var freqAlphabets = function (s) {
      let res = [];
      for (let i = 0; i < s.length; i++) {
        if (s[i] === '#') {
          res.pop();
          res.pop();
          res.push(String.fromCharCode(+(s[i - 2] + s[i - 1]) + 96));
        } else {
          res.push(String.fromCharCode(+s[i] + 96))
        }
      }
      return res.join('');
    };
    console.log(freqAlphabets("10#11#12"));
  </script>
</body>

</html>